Automatic update of the image in the UV/image Editor
UPDATE: I've noticed that some of what I wrote here originally is unnecessary.  If the Slot mode in Texture Paint is left set to 'Material'  you can simply select a node in Cycles and quickly enter and exit Texture Paint mode to have the texture appear in the UV Image Editor.



That's an unfortunate design choice for the UV Image Editor in Cycles. It could change if people come up with some clear and well constructed reasons for changing the current system so the BF devs might feel motivated to change this.  

There is a way around the issue now that Slots have been added to Texture Paint.

(Incidently, to make the slots tab show on the main tab you just have to shift+ click the handle to pin it so the tab is always visible)



The following image shows where to find the selector when default settings are used.

Just be sure to change the Painting Mode to Image since the default is Material.